In view of their physical properties, GaN-based high electron mobility transistors (GaN-HEMTs) are expected, as high voltage devices capable of operating at a high speed, to be applied to millimeter-wave radar systems, systems for wireless base stations, server systems, for example.
However, when holes generated by impact ionization in a high electric field accumulate in an electron transit layer (channel), reduction in the breakdown voltage of the device, variation in the drain conductance characteristics due to the kink effect, and deterioration of the switching speed and so forth occur.
These issues can be effectively resolved by extracting holes generated by impact ionization from the channel.
There have been proposed three hole extracting structures, as depicted in FIGS. 14A-14C, for example.
There is a device structure having a p-type GaN layer, a GaN layer, and an AlGaN layer, as illustrated in FIG. 14A, wherein a source electrode, a drain electrode, and a gate electrode are formed on the AlGaN layer, and a hole extracting electrode is disposed on the back surface side of the device (first technique). This is the structure wherein holes are extracted from the back surface side via the p-type GaN layer. Additionally, there is a structure wherein a hole extracting electrode is disposed on the back surface side of the GaN layer, without providing a p-type GaN layer (second technique).
Furthermore, there is a device structure having a GaN layer and an AlGaN layer above a substrate, as illustrated in FIG. 14B, wherein a hole extracting electrode is provided on the GaN layer exposed by etching the layers to a depth deeper than the interface between the AlGaN layer and the GaN layer by, for example, dry etching and so forth (third technique). There is also a device structure having an AlGaN buffer layer between a substrate and a GaN layer (AlGaN/GaN/AlGaN device structure), wherein a hole extracting electrode is provided on the GaN layer exposed by etching the layers to a depth near the interface between the GaN layer and the AlGaN layer where two-dimensional hole gas (2DHG) is formed (fourth technique). In this case, the hole extracting electrode is provided on the GaN layer remained on the AlGaN layer.
Even further, there is a device structure having a GaN layer and an AlGaN layer on a substrate, as illustrated in FIG. 14C, wherein p-type impurities (such as magnesium (Mg), for example) are ion implanted, a rapid thermal anneal (RTA) is performed at a high temperature of 1000° C. or higher, and a hole extracting electrode is provided on the activated p-type region (fifth technique).